1. Field of the Invention
The present invention relates to an apparatus for charging a rechargeable battery. More specifically, the present invention relates to an apparatus for charging a gas tight sealed rechargeable battery which exhibits a charging voltage characteristic having a peak shortly before the battery reaches a fully charged state, whereupon the voltage gradually decreases until the battery reaches a fully charged state.
2. Description of the Prior Art
Conventionally the so-called rapid charging for charging a battery with an increased charging current value as well as the so-called trickle charging has been effected in a rechargeable battery. In effecting such rapid charging, it is necessary to detect a charged state of a battery being charged and to interrupt a charging current in order to avoid overcharging.
On the other hand, it has been well known that a gas tight sealed battery such as a nickel-cadmium battery exhibits a charging characteristic as shown in FIG. 1. The charging quantity characteristic of such gas tight sealed battery exhibits a change as shown by the curve B in FIG. 1, in which the saturated state is a fully charged state of the battery. The charging voltage characteristic of such gas tight sealed battery is shown by the curve A in FIG. 1, which exhibits an increase until a peak (as denoted as a) shortly before a fully charged state is reached and thereafter exhibits a gradual decrease until a fully charged state is reached. Therefore, if and when a simple approach is employed to interrupt a charging current by detecting a charged state of a battery in rapid charging as described previously, an overcharged state or an undercharged state results. In order to avoid such state and in order to properly charge a battery, therefore, various approaches were proposed and put into practical use.
One of the prior art approach of interest to the present invention is disclosed in U.S. Pat. No. 4,134,056 issued Jan. 9, 1979 to the same assignee as the present invention. The charging apparatus disclosed in the referenced United States patent was considerably advantageous as compared with the prior art at that time. Nevertheless, the charging apparatus disclosed in the referenced U.S. patent still involves a problem to be solved. More specifically, the apparatus disclosed in the referenced United States patent is adapted to interrupt a charging current upon detection of a charged state of a rechargeable battery based on the level of a differentiated output voltage of a charged voltage. Therefore, the characteristic of the differential output voltage is diversified because of the diversified characteristic of batteries and accordingly it is difficult to secure a predetermined charging quantity (%) for each of the batteries having different characteristics.
Another prior art approach of interest to the present invention is disclosed in Japanese Pat. No. 960,966 published for opposition Oct. 28, 1978 and granted June 28, 1979. The charging apparatus disclosed in the referenced Japanese patent is adapted such that a variation of the voltage per unit time of the charging voltage characteristic curve, i.e. a differentiation coefficient of the charging voltage characteristic is detected, so that the charging current is interrupted when the differentiation coefficient reaches zero or a given minus value. However, even the charging apparatus disclosed in the referenced Japanese patent still involves a problem to be solved. More specifically, as seen from FIG. 1, when the charging current is interrupted at the differentiated value being zero, the differentiated value being zero means the peak point a on the curve A in FIG. 1, i.e. the timing t.sub.1. As further seen from the curve B, the timing t.sub.1 is not yet a fully charged state of the battery, wherein the battery has not been fully charged. Nevertheless, even if it is adapted such that the charging current is interrupted when the differentiated value becomes a given minus value, a predetermined charging quantity cannot be necessarily secured, i.e. the charging voltage characteristic largely fluctuates depending on the ambient temperature. On the other hand, according to the referenced Japanese patent, such differentiated value is detected at predetermined time intervals. Therefore, if and when the charging voltage characteristic largely fluctuates, the detecting time point is largely different and in an extreme case, it could happen that such an arbitrary minus value of the differentiated value cannot be detected and the charging operation cannot be terminated, with the result that the battery is overcharged.